As an electrode pad for use on a living organism of the prior art, e.g. the electrode pad disclosed in the Japanese patent application laid-open No. 2006-231020 is known, which was formerly proposed by the applicant of the present application. The electrode pad for use on a living organism has a plurality of electrodes having mutually different area, each contacts a surface of a skin at a predetermined position of a living organism through conductive gel. In contact of the plurality of electrode pads with the surface of the skin on the living organism, the electrocardiograph determines a basic electrocardiographic signal by evaluating a difference of the signals from either one electrode of each of the electrode pads, while determining a body-motion noise signal by evaluating a difference of the signals from a relatively narrower electrode and a relatively wider electrode of each of the electrode pads, then enables to determine the electrocardiographic signal without being interrupted by a body-motion noise, by eliminating the low-frequency component of the body-motion noise signal from the basic electrocardiographic signal. Incidentally, the body-motion noise signal is assumed to be generated due to fluctuating of polarization potential or impedance between an electrode and conductive gel or between conductive gel and a surface of the skin.